1. Field of the Invention
The present invention relates in general to an apparatus provided for the internal welding of joints between pipes, and more specifically to apparatus for the welding of an edge joint between a branch, first, pipe and a main, second, pipe which pipes together form a pipe branch, wherein the cross section of the first pipe is smaller than that of the second pipe and wherein at least the first pipe is straight.
2. Description of the Prior Art
It is known to provide welding apparatus equipped with a nozzle holder introduced into the first pipe and capable of being displaced axially by a suitable actuating mechanism. The nozzle holder can have at its front end, which projects through the first pipe and into the second pipe, a gas nozzle including a non-consumable stick electrode that forms with the axis of the first pipe an acute angle, and which is connected to a support holder for a frame attached to the first pipe and incapable of angular movement relative to the latter pipe. The support holder together with the nozzle holder can be caused to rotate by a turning arrangement relative to the frame about the axis of the first pipe.
There is described, for example, in U.S. Pat. No. 4,104,499, an electrode capable of rotating about the axis of the first pipe, with the electrode having a tip that follows a circular path. Production tolerances in relation to the pipe frequently give rise to deviations from the precise, circular cylindrical form of the pipe, which deviations can cause variations in the travel length of the arc to have an adverse effect on the quality of the welding. In conjunction with the welding of pipe branches, the electrode must also describe a movement along the axis of the first pipe such that it may follow the line of intersection between the two pipes which make up the welded joint. It is proposed in the aforementioned publication that this axial movement be achieved by means of a cam control system, which also presupposes the extremely high cylindrical accuracy of the pipes with regard to their geometrical form. To compensate for objectionable variations in the dimensions of the pipe, the cam control can be modified to operate in accordance with the branch first pipe in question. A control arrangement which includes such comprehensive modifications in connection with any change to the dimensions of the workpiece substantially increases the difficulty of satisfactory production. Moreover, the complex design of the involved cam makes the resulting cam control system an expensive feature, which must additionally be made available in a large number of variants suited to the actual dimensions of the pipe.
In connection with the internal welding of a branch first pipe, it is particularly desirable that the completed weld should not require any subsequent finishing. Finishing is, in practice, a complicated operation in connection with the welding of small pipes, where the weld is accessible only with difficulty and the visual inspection of the deposited weld is difficult to execute.